Nanoporous dielectrics are some of the most promising new materials for semiconductor fabrication. These dielectric materials contain a solid structure, for example of silica, which is permeated with an interconnected network of pores having diameters typically on the order of a few nanometers. These materials may be formed with extremely high porosities, with corresponding dielectric constants typically less than half the dielectric constant of dense silica. And yet despite their high porosity, it has been found that nanoporous dielectrics may be fabricated which have high strength and excellent comparability with most existing semiconductor fabrication processes. Thus nanoporous dielectrics offer a viable low-dielectric constant replacement for common semiconductor dielectrics such as dense silica.
The preferred method for forming nanoporous dielectrics is through the use of sol-gel techniques. The word sol-gel does not describe a product but a reaction mechanism whereby a sol, which is a colloidal suspension of solid particles in a liquid, transforms into a gel due to growth and interconnection of the solid particles. One theory is that through continued reactions within the sol, one or more molecules in the sol may eventually reach macroscopic dimensions so that it/they form a solid network which extends substantially throughout the sol. At this point (called the gel point), the substance is said to be a gel By this definition, a gel is a substance that contains a continuous solid skeleton enclosing a continuous liquid phase. As the skeleton is porous, the term "gel" as used herein means an open-pored solid structure enclosing a pore fluid.
One method of forming a sol is through hydrolysis and condensation reactions, which can cause a multifunctional monomer in a solution to polymerize into relatively large, highly branched particles. Many monomers suitable for such polymerization are metal alkoxides. For example, a tetraethylorthosilicate (TEOS) monomer may be partially hydrolyzed in water by the reaction EQU Si(OEt).sub.4 +H.sub.2 O.fwdarw.HO--Si(OEt).sub.3 +EtOH
Reaction conditions may be controlled such that, on the average, each monomer undergoes a desired number of hydrolysis reactions to partially or fully hydrolyze the monomer. TEOS which has been fully hydrolyzed becomes Si(OH).sub.4. Once a molecule has been at least partially hydrolyzed, two molecules can then link together in a condensation reaction, such as EQU (OEt).sub.3 Si--OH+HO--Si(OH).sub.3 .fwdarw.(OEt).sub.3 Si--O--Si(OH).sub.3 +H.sub.2 O
or EQU (OEt).sub.3 Si--OEt+HO--Si(OEt).sub.3 .fwdarw.(OEt).sub.3 Si--O--Si(OEt).sub.3 +EtOH
to form an oligomer and liberate a molecule of water or ethanol. The Si--O--Si configuration in the oligomer formed by these reactions has three sites available at each end for further hydrolysis and condensation. Thus, additional monomers or oligomers can be added to this molecule in a somewhat random fashion to create a highly branched polymeric molecule from literally thousands of monomers. An oligomerized metal alkoxide, as defined herein, comprises molecules formed from at least two alkoxide monomers, but does not comprise a gel.
Sol-gel reactions forms the basis for xerogel and aerogel film deposition. In a typical thin film xerogel process, an ungelled precursor sol may be applied (e.g., spray coated, dip-coated, or spin-coated) to a substrate to form a thin film on the order of several microns or less in thickness, gelled, and dried. The precursor sol often comprises a stock solution and a solvent, and possibly also a gelation catalyst that modifies the pH of the precursor sol in order to speed gelation. During and after coating, the volatile components in the sol thin film are usually allowed to rapidly evaporate. Thus, the deposition, gelation, and drying phases may take place simultaneously (at least to some degree) as the film collapses rapidly to a dense film. In contrast, an aerogel process differs from a xerogel process largely by avoiding pore collapse during drying of the wet gel. Some methods for avoiding pore collapse include wet gel treatment with condensation-inhibiting modifying agents (as described in U.S. Pat. No. 5,470,802, A Low Dielectric Constant Material For Electronics Applications, issued on Nov. 28, 1995 to Gnade, Cho and Smith), supercritical pore fluid extraction, and freeze-drying.
Aerogels are the preferable of the two dried gel materials for semiconductor thin film dielectric applications. Typical thin film xerogel methods produce films having limited porosity (up to 60% with large pore sizes, but generally substantially less than 50% with pore sizes useful in submicron semiconductor fabrication). An aerogel thin film, on the other hand, may be formed with almost any desired porosity coupled with a very fine pore size. Preferably, for semiconductor applications these nanoporous materials have average pore sizes less than 50 nm (and more preferably less than 10 nanometers and still more preferably less than 5 nanometers). The nanoporous inorganic dielectrics include the nanoporous metal oxides, particularly nanoporous silica.